Methods and apparatus for texturising such extruded yarns are well known and it is desirable to maintain as constant a bulk level of the crimp-textured yarn as possible since variation can result in shading of the colour of the finished product, such as carpet or fabric. One particular method of fluid jet texturising of a yarn and controlling the crimp is described in British patent No. 1,422,949 in which the crimped yarn is formed into an elongated plug, the position of the leading end of which is monitored about a predetermined datum position, variations therefrom indicating changes in the bulk level of the crimped yarn. If the plug is too long it indicates that there is insufficient bulk and therefore the temperature of the jet, i.e. the temperature of the fluid (e.g. air) in the jet, is increased so as to increase the plasticity of the yarn and hence make it crimp more readily. Conversely, if the plug is too short it indicates the requirement for a reduction in the bulk and hence the temperature of the jet is decreased. Various alternative sensing arrangements are described in the above patent in which the end of the plug is sensed in the stack following the crimp chamber or on a crimping or cooling drum.
Other parameters may be adjusted to give a similar result. These are, in particular, the velocity and/or pressure of the fluid in the jet and the temperature of the rollers upstream of the jet which impart an initial stretch to the yarn, or heat it further subsequent to stretching, or the temperature of a plate or plates which may also be used for heating the yarn.
It will be understood from the foregoing that the position of the leading end of the crimp plug in relation to the datum position is the dominant factor in maintaining a substantially constant bulk level of the crimp-textured yarn, that is to say, a predetermined degree of bulk. However, if a break in the yarn occurs, especially in the length of yarn between the crimp plug and the winder, the plug length will continue to grow and the leading end of the plug will pass beyond the datum position since yarn will not then be actively pulled away. Assuming that the parameter being controlled is the jet temperature, this will result in increasing power to the heating element for the jet. Therefore when the yarn is subsequently threaded up again and winding re-commences, it may take an appreciable time for the end of the plug to re-stabilise at the sensor position, leading to production of yarn with incorrect bulk level. Since the yarn speed may be in the region of 1500 metres/minute, a very considerable length of yarn would be produced with much too high a bulk level and if allowed to be wound onto the package would result in colouring problems with the finished product. To avoid this, the normal practice is to use an aspirator gun which aspirates the faulty yarn into a reservoir for subsequent disposal to waste. When threading up again, the operator establishes the plug end in the vicinity of the datum position and operation can then resume, the sensor control restoring the leading end of the plug to the correct position within a relatively short space of time.
Conversely, if a yarn break occurs upstream of the texturising jet, the yarn will be withdrawn from the downstream end of the plug while none will be added to the upstream end, i.e. from the texturising jet, hence resulting in an incorrect signal from the sensor that the crimp is too great and resulting in reduction of power to the jet heating element to its minimum. When rethreading, it may again take some time for the leading end of the crimp plug to re-stabilise at its datum position, again resulting in the production of inconsistently bulked yarn, this time with too little crimp. This same condition will apply when an aspirator gun is used for sucking away a broken end, if in so doing the leading end of the plug falls behind the datum position.